DE4319741A1 - Process for producing refractory products based on magnesium oxide - Google Patents

Abstract

The invention relates to a process for producing refractory products based on magnesium oxide. In this process, porous, granular aluminium oxide materials having a particle size of at least 0.5 mm are used and added to the mixtures of the granular magnesia materials. The porous, granular aluminium oxide materials are obtained by a granulation process, preferably from calcined fine alumina. The granular materials of Al2O3 can contain additional additives.

Description

Due to their high thermal expansion, basic refractory products of magnesium oxide have a very poor thermal shock resistance (TWB). To improve the TWB, the microstructure is built up heterogeneously by additives with a lower thermal expansion (chromite or spinel). In recent years, magnesia spinel stones have been used successfully for lining furnace areas, particularly in the zero-neck zones of the rotary kilns of the cement industry. The main advantage of these stones is the higher TWB and the associated duration of use. In addition, the disposal problem associated with the Cr ⁶⁺ ion does not occur / P. Bartha; Lecture Intern. Refractory Colloquium Aachen 1983 /.

Since the spinel formation of dense Al₂O₃ and dense magnesia always brings a volume expansion, it often comes to a Loosening of the structure of magnesia stones with Clay additive. As a result, the Magnesiastein with a Al₂0₃- Addition a very low mechanical strength at high Temperatures that are detrimental to the behavior of such Stones against abrasion and mechanical stress is. In / US 3, 333, 971 / it was proposed to increase the volume the spinel formation by a ZrO₂ addition of 3 to 15% suppress. However, the expensive zirconia precipitates in the production costs of the stones down.

Very fine alumina with a mean diameter smaller than 5 microns was added to the magnesia grains / OS 0001 327 /. Such mixtures according to the instructions showed no great Dimensional change after firing at 1400 ° C. Nevertheless, these have Stones still provide low mechanical strength elevated temperatures.

The problem of volume growth due to spinel formation Stone making is also through the use pre-made spinels solved.  

The spinel synthesis takes place extra in a sintered or Melting process and is very expensive.

The invention has for its object to provide a method for Production of basic refractory products Magnesium oxide base with alumina additive to develop the a high resistance to thermal shock load and have good mechanical strength at the same time should.

As magnesium oxide component come sintered and / or Melt magnesia used. According to the invention are porous Aluminum oxide grains with a size of at least 0.5 mm added to the mixtures of magnesium oxide grains. Furthermore, the invention provides that fine-grained Granulated alumina to porous alumina granules before it is added to the mixtures of sintered and / or Fused magnesia grains is introduced. Of the many commercially available grades of alumina z. B. calcined Alumina, tabular earth or fused alumina is the calcined alumina to be preferred, because these have a high Has reactivity, which is favorable to the sintering effect. In addition, the calcined clay is less expensive as tabular soil and fused corundum. According to their production For example, calcined clay is in the form of powder. To be porous Aluminiumoxidkörnungen the desired grain sizes get fine grained clay z. B. by the Compacting compacted into larger moldings and then comminuted into corresponding particle fractions. The strength problem with the Magnesia stones with Alumina additives seem not only the result of Volume expansion during spinel formation, but more the Consequence of the occurring tensions between the binding matrix and the coarse grains. This fact is thereby to explain that because of high fineness of the added alumina the matrix is to be regarded as homogeneous. Because the matrix is one has much smaller thermal expansion than the magnesia creates a tension at the Magnesiakorngrenzen the Binding matrix as the stones cool down from the burn Room temperature, which causes a reduction in strength. There  According to the invention, the matrix of the same magnesium oxide like the main part of the grain, can be a better one Compound of magnesium oxide grain with the matrix arise as in the simple admixture of the fine alumina to the magnesium oxide grains is possible, so that a higher Strength of the refractory composition is achieved. At the same time a good TWB is characterized by a high inhomogeneity of the stoneware when adding the porous Obtained alumina grains.

The alumina granules are initially porous, so that at the spinel formation occurring volume growth effect can be intercepted. The according to the invention Molded bodies have a good TWB and markedly higher cold and hot strengths than the stones same chemical composition, in which one fine-grained alumina the magnesium oxide grains admixed. Surprisingly, the stones after the Invention no increased porosity. There is no additional cement clinker melt or slag infiltration to be expected in the stones during use. With the Processes according to the invention can be refractory products longer life can be produced.

The stones according to the invention are composed of 85 to 97% by weight. Magnesium oxide grains and 15 to 3 wt.% Porous Alumina particles together. Of course, one can Part of the magnesium oxide component by a corresponding Share of other TWB-improving raw materials z. Spinel be replaced.

Also, the invention provides that the Magnesium oxide component has an MgO content of at least 95% by weight and the alumina raw material should have an Al₂O₃- Share of at least 97 wt.% Has. With this purity the magnesia component and the alumina raw material is a high fire resistance of the stones made from it guaranteed.

In another embodiment, the porous Aluminum oxide granules preferably in the particle size range 1.0 used up to 2.5 mm. This will get the finished stones  optimal properties regarding the TWB, the mechanical Strength and corrosion resistance.

The invention also provides that the granules of finely divided Al₂O₃ raw materials other components such preferably the oxides ZrO₂, TiO₂, Fe₂O₃ or mixtures thereof may contain. As is known, the spinel grain becomes intensely attacked by calcareous substances. By According to the invention, first the amount of alumina with additive homogenized and then granulated, the chemical Composition of the spinel grain formed in the stone fire change in a simple way. This can be a better one chemical corrosion resistance or more favorable Approach behavior of the stones are achieved. After this The process of the invention can be the effect of additional admixtures to improve the properties reach effectively even with small quantities.

The invention will be explained in more detail by means of examples.

example

The compositions and properties of the stones are in Table 1 together with comparative samples.

The sintered magnesia used consists of:

MgO 98.6% by weight; CaO 0.81% by weight; SiO₂ 0.1% by weight; Fe₂O₃ 0.17% by weight; Al₂O₃ 0.2% by weight.

The alumina for the production of alumina granules has the following features:

Al₂O₃ content 98.75% by weight Na₂O content 0.06% by weight

Maximum particle size (d _max ) 40 μm 50% by weight of the clay less than 5 μm (d = 5 μm).

From this clay 4 different types of grainy with or without admixtures, proportionate to the mass of Grit made:

The stones 1 to 4 were produced according to the invention. The remaining stones 5 to 7 are used for comparison with the Stones 1 to 4.

The stone 5 is a pure magnesia stone without additives. The stone 6 has the same chemical composition as the Stone 1, but with a fine above Alumina additive instead of granules.

The stone 7 was not added clay, but a Sintered spinel of the composition (33 wt% MgO; 66 wt% Al₂O₃). This stone is a magnesia spinel stone of the 2. Generation.

All mixtures become more aqueous with sufficient amount Magnesiumsulfitablauge moistened and after the usual Forming process pressed to stone at 120 MPa, then dried and then at 1700 ° C and a holding time burned for 4 hours.

The bulk density and the open porosity were according to DIN 51065 and DIN 51056 tested.

The hot bending strength determination is carried out on samples of the Dimensions (25 × 25 × 150 mm³) at 1400 ° C and 1 h hold time carried out.

The dynamic modulus of elasticity (modulus of elasticity) is applied to samples of the Dimensions (25 × 25 × 150 mm³) under Bending vibration excitation determined.

The measure of thermal shock resistance (TWB) is the Number of quenching of cylinder samples (diameter × Height = 59 mm × 50 mm) with water (950 ° C ↔ 25 ° C) until Fracture.  

The stones made with an applied tablet Portland cement clinker will be at 1600 ° C and a holding time treated by 10 hours. This will be the contact reaction and investigated the infiltration of cement clinker melt.  

Table 1

Claims (7)

1. A process for the preparation of refractory products based on magnesium oxide, characterized in that as magnesium oxide sintered and / or fused magnesia is used and the mixture of the Magnesiumoxidkörnungen porous Aluminiumoxidkörnungen having a size of at least 0.5 mm are added. 2. The method according to claim 1, characterized in that the porous Aluminiumoxidkörnungen greater than 0.5 mm fine-grained aluminum oxides produced by granulation become. 3. Process according to claims 1 and 2, characterized characterized in that the proportion of the magnesium oxide grain 85 to 97% by weight and the amount of alumina granules 15 to 3 wt.% In the mixture. 4. Process according to claims 1 to 3, characterized characterized in that the magnesium oxide granules have a MgO Have content of at least 95 wt.% Own. 5. Process according to claims 1 to 4, characterized characterized in that the alumina raw material is an Al₂O₃- Content of at least 97 wt.% Has. 6. Process according to claims 2 and 5, characterized characterized in that the grain size of the recovered Alminiumoxidgranulats 1.0 to 2.5 mm. 7. Process according to claims 2, 5 and 6, characterized characterized in that the alumina granules further Components such as preferably the oxides ZrO₂, TiO₂, Fe₂O₃ or may contain their mixture.